This invention relates generally to methods and apparatus for recharging batteries including but not limited to NiCad batteries or NiMH batteries. More particularly, the invention relates to methods and apparatus for recharging batteries in an environment of variable ambient temperatures.
Rechargeable batteries, such as NiCad (nickel cadmium) batteries or NiMH (nickel metal hydride) batteries are known to undergo a recharge cycle of first and second charging phases. A first phase is referred to as a coulombic charging phase. During the coulombic charging phase most of the charge applied by the charging current is stored in the form of electro-chemical energy. At the end of the coulombic charging phase most of the charge holding capacity of a rechargeable battery has been restored to its charged state. Further charging extends into a second phase which is known as an overcharge phase. During the overcharge phase of a charging cycle the charging current generates oxygen gas at one of the electrodes. During the overcharge phase of the recharge cycle only a small portion of the charging energy becomes stored in the form of electro-chemical energy. Complex or "smart" battery chargers have been devised to add further charge to any battery which is at that time already substantially charged. However, charging a battery in the overcharge phase holds dangers to the battery and should generally be avoided. Charging batteries at excessive charging rates while they are in the overcharge phase may cause gas pressures to build up, in spite of venting provisions built into such batteries, such that carelessly charged batteries may become permanently damaged.
Simple battery chargers, though of good quality, seek to avoid charging batteries into the overcharge phase, by determining the onset of the overcharge phase which is marked by a distinct temperature rise in the battery. The temperature rise may be determined by a typical temperature sensor. Such a sensor, either a thermistor or a semiconductive temperature sensing element may preferably be built directly into a battery pack. Such a battery pack having a temperature monitor may be recognized by three battery terminals instead or two. Two of the terminals are power and ground terminals, and a third terminal provides a temperature-dependent signal, referenced to ground. The temperature-dependent signal may be a voltage signal, for example, to be used as an input signal to a battery charging device. In the interest of saving a battery from becoming damaged by overcharging, batteries are often not charged to their capacity. Even though precautions may be taken to avoid damage to a battery by an overcharge, an apparent problem has now been recognized, according to which the point at which the overcharge phase of a battery begins apparently may not be as readily determined as it is generally believed to be the case.